Thermally-conductive, electrically non-conductive heat transfer material and articles made thereof

ABSTRACT

A heat transfer material comprised of a polymeric material and a nitride or oxide is provided and is thermally conductive, but electrically non-conductive. The polymeric material may be silicone rubber, and the nitride or oxide may be aluminum nitride, boron nitride, silicon nitride, aluminum oxide or beryllium oxide. The material is made flexible by adding plasticizer and remains flexible after an extended period of use. A dielectric jacket for an electrical heating cable is thermally conductive and remains flexible so that it can be reused. The heat transfer material can be used on heating devices such as steam tubes and panels, where the heat transfer material does not bond to or adhere to the heated surface, which allows the heating device to be reused.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/165,441, filed Jun. 7, 2002, which is a divisional of U.S.application Ser. No. 09/353,675, filed Jul. 15, 1999, which claimspriority to U.S. Provisional Patent Application No. 60/092,943, filedJul. 15, 1998. This application claims the benefit of and priority toeach of these prior applications, all of which are herein incorporatedby reference in their entirety for all purposes.

STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to heat transfer material andmore particularly to one that is thermally conductive, but electricallynon-conductive. Articles made with the material include a dielectricjacket for a heating cable, and a thermally conductive, but electricallynon-conductive, jacket for a steam/fluid tracer tube as well asthermally conductive strips for bridges between steam/fluid/electricallyheated tubes and process piping, vessels, and equipment.

2. Description of the Related Art

The use of thermally conductive materials in heat tracing applicationsis known in the art. As early as 1954, filled thermally conductivematerials were being commercially used in industrial heat tracingapplications. Early heat transfer materials for heat tracing used carbonbased fillers, such as graphite, loaded into a receiving base materialsuch as sodium silicate, epoxy, etc. These materials were applied inpaste form to the exterior of a tube through which steam was passed. Thepassage of steam through the tube caused the water in the sodiumsilicate to evaporate. This resulted in the heat transfer materialhardening and thus permanently and physically bonding the steam tube tothe process pipe to which it was mounted. This physical bonding enhancedthe heat transfer between the steam tube and the process pipe and thusresulted in much higher maintenance temperatures on the process pipethan would be experienced by traditional steam tracing methods using noheat transfer material for a given steam/fluid temperature.

In 1974, Bilbro et al. obtained U.S. Pat. No. 3,834,458 for a new heattransfer material which achieved similar results as prior heat transfermaterials but allowed for a partially cured conductive material to besnapped in place over the tube and then covered with a containingchannel. The advantage here was the heat transfer material became moltenand flowed to fill air gaps after steam was passed through the tube. Theinstallation of the heat transfer material was cleaner and faster. Theconductivities of the cured heat transfer material of the '458 patentwere only slightly less than the previous paste-like heat transfermaterials. The heat transfer material disclosed in the '458 patent wasalso extensively used with electric heat tracers by extruding the heattransfer material onto the electric cable at the factory and thenshipping the electric heat tracer to the field on a reel. In the field,the electric cable with the extruded heat transfer cement was installedon a pipe and again covered with a channel.

In recent years, certain applications have been identified where it isnot possible to keep the extruded heat transfer cement material, asdisclosed in the '458 patent, always beneath a channel. One specificapplication is the rail heating application. Specifically, when usedwith rail heating, the electric cable heater has to leave the rail atexpansion joints and then after a one or two foot loop return to heatthe rail. The prior art heating cable included an extruded thermallyconductive and electrically conductive heat transfer material. The heattransfer material contained carbon black, which provides the requiredthermal conductivity, however, it is also highly electricallyconductive.

Since the prior art heat transfer material was electrically conductive,it posed a hazard for electrical shock. Thus, in the past, a thinsilicone rubber jacket has been placed around the extruded heat transfercement material to retain its shape at the excursion points of theheater cable from the rail. Since the rails in many cases wereelectrically alive (480 to 800 volts DC or AC potential), the siliconejacket material provided electrical insulation—should anyone brushagainst these loop arounds. Materials other than silicone have also beenused for this purpose, one of which is described in U.S. Pat. No.4,391,425, issued to Keep.

Many other applications also require dielectric jackets, so electricalconductivity of prior art heat transfer materials is often a problem.Due to the composition of the prior art heat transfer material used, theheat transfer material would cure and harden when placed into service.Consequently, prior art heating cable was typically not reusable afterit was removed from a heated surface because it became hard and brittleduring service. In the rail heating application, when rail replacementwas necessary, it also became necessary to replace the heating cable.

Similarly, heat transfer material that has been extruded onto asteam/fluid tracer tube and installed under a channel typically cannotbe subsequently removed and reinstalled without damaging the heattransfer material. Most prior art heat transfer materials forsteam/fluid tracing bond or adhere to some extent to the heated surfacewhen in service, which again prevents reuse. Where heat transfermaterial has been used between two tubes, which have high expansionforces, the expansion forces have caused the material to crack.

BRIEF SUMMARY OF THE INVENTION

A thermally conductive, but electrically non-conductive, heat transfermaterial is provided according to the present invention. For example, ajacket or insulation layer is provided for heating cables for railheating applications, electric heating and power cables, jacketedsteam/fluid tracer tubes, and removable/reusable thermal bridge stripsfor heat tracing tubes. The thermally conductive, but electricallynon-conductive, articles so made are mechanically sturdy, but flexible.Cable, tubes, bridge strips and similar articles can be shipped on areel to the final destination. A thermally conductive material for heattransfer devices is provided that retains flexibility after use, whichhas dielectric properties. Articles made with the present heat transfermaterial do not pose an electrical shock; do not become hard and brittleafter use; and do not become bonded to the surface. Yet, the materialmeets thermal conductivity requirements.

The thermally conductive, electrically non-conductive compositioncomprises a polymeric material, such as silicone rubber, and a nitrideand/or oxide compound as a filler material. Suitable nitride and/oroxide compounds include, but are not limited to, aluminum nitride, boronnitride, silicon nitride, aluminum oxide and beryllium oxide. Compoundsthat are chemically or physically similar to the specified nitride andoxide compounds may be suitable as well. Preferably, additionalplasticizer additives are included to increase the flexibility of thejacket material. The jacket material of the present invention has athermal conductivity that approaches the thermal conductivities of priorart heat transfer materials, is not electrically conductive, and remainsflexible at temperature exposures up to and exceeding 450° F. and doesnot harden or adhere to the substrate.

A heating cable has a thermally conductive, electrically non-conductivejacket. Such a cable can be installed on a third rail that is usuallyelectrically alive with 480 volts to 800 volts DC or AC potential. Theheating cable with a jacket according to the present invention can beinstalled on a live third rail without a danger of electrical shock tothe installer. The thermally conductive, electrically non-conductivejacket will not form a galvanic corrosion (cell) on the carbon steelthird rail. The jacket can be extruded onto the cable duringmanufacture.

A heating cable according to the present invention, with a thermallyconductive, electrically non-conductive jacket, can be used in electricheat tracing applications, where reduced element and conductor operatingtemperatures are advantageous. A composition of material according tothe present invention is also useful as a thermally conductive jacketfor steam/fluid tube tracers or panels or thermal bridge strips betweentracers and the heated surface which allows high heat transfer rates butallows the tracer to be removed and reapplied without sustaining damageto the heat transfer material.

It is desirable to have an improved thermally conductive, electricallynon-conductive jacket for heating cable for rail heating applications.It is further desirable to have a thermally conductive jacket for a heattransfer element that retains flexibility after use. Many types ofheating and power cable products require dielectric jackets. It would beadvantageous for these heating cables to be jacketed with a highlythermally conductive material in order to reduce the innerconductor/element operating temperature. As these jackets are dielectricjackets, they should remain essentially electrically nonconductive. Aheat transfer material according to the present invention or jacket,sheath, strip, insulator or covering made of it addresses these desires.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The objects, advantages, and features of the invention will become moreapparent by reference to the drawings which are appended hereto andwherein like numerals indicate like parts and wherein an illustratedembodiment of the invention is shown, in which:

FIG. 1 is a sectional view of a heating cable having a thermallyconductive, electrically non-conductive jacket, according to the presentinvention thereon;

FIG. 2 shows the jacket material of the present invention used with aheating cable and mounted on a rail;

FIG. 3 is a sectional view of the installation of the steam/fluid tracerwith the jacket of the present invention on a process pipe;

FIG. 4 shows an external steam/fluid panel in service on a tank, pipe orvessel;

FIG. 5 shows a partial cross section of the panel and heated componentof FIG. 4 with a strip of heat transfer material therebetween, accordingto the present invention;

FIG. 6 is a sectional view of a skin-effect heat tube on a process pipewith heat transfer material therebetween, according to the presentinvention; and

FIG. 7 is a chart illustrating the performance of a heat transfermaterial according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A thermally conductive, but electrically non-conductive, heat transfermaterial is useful as a jacket or insulation layer for a heating device.The heating device can be, for example, a steam or electrically heatedtube, panel, or an electric heating device. A jacket according to thepresent invention can be extruded onto a heating cable, which is aheating device that is particularly suitable for rail heatingapplications in addition to numerous other applications. The thermallyconductive, but electrically non-conductive, cable so made ismechanically sturdy but flexible so that it can be shipped on a reel tothe final destination.

A thermally conductive, electrically non-conductive, heat transfermaterial according to the present invention is comprised of a polymericmaterial, such as a silicone rubber, and a filler material that addsthermal conductivity without adding electrical conductivity. Thepolymeric material is typically silicone rubber, but may be, forexample, silicone gels, polyethylene, polypropylene, an elastomer,natural or synthetic rubber, or epoxy. Examples of the filler materialinclude aluminum nitride (AlN), boron nitride (BN), silicon nitride(Si3N4), aluminum oxide (Al₂O₃), and beryllium oxide (BeO), butcompounds or materials exhibiting similar chemical or physicalproperties may also be suitable. The filler is preferably provided inthe range of approximately 30% to 60% by volume and more preferably inthe range of 40% to 50% by volume.

In order that the jacket of the present invention be flexible,additional plasticizer additives are included. The plasticizer additivesare preferably, but not necessarily, silicone based modifiers such asSilastic HA-2 provided by Dow Corning (STI) Kendalhall, Ind., U.S.A. Theplasticizer loading is preferably in the range of approximately 0% toabout 15% of filler weight and more preferably in the range of about 5%to about 10% of the filler weight.

One heat transfer material according to this invention is a compoundedmixture of silicone rubber and aluminum nitride (AlN) as the filler. Thepercent filler loading of aluminum nitride is preferably in the range ofapproximately 30% to 60% by volume and more preferably in the range of40% to 50% by volume.

The plasticizer loading is preferably in the range of approximately 0%to 15% of filler weight and more preferably in the range of 5% to 10% ofthe filler weight. Other property enhancers, such as fire retardants andultra violet inhibitors, may also be used without substantially changingthe material's heat transfer performance. Adhesive materials such assilicones and other similar materials can be used to bond, stick oradhere this compound to a substrate if desired.

With reference to FIG. 1, a heating cable 10 according to the presentinvention is illustrated in cross section. Cable 10 has first and secondelectrical conductors 12 and 14, respectively, which are surrounded byan electrically insulating material 12 a and 14 a, respectively. Asheath 18 covers electrical conductors 12 and 14, and a jacket 20 coverssheath 18. Jacket 20 is illustrative of the heat transfer material ofthe present invention. Jacket 20 is a dielectric material, which iselectrically non-conducting. Thus, conductors 12 and 14 may have a highvoltage potential, yet jacket 20 allows heater cable 10 to be safelytouched without electrical shock.

Conductors 12 and 14 generate heat using electrical resistivity, andjacket 20 conducts that heat to a surface that is to be heated. In aprior art cable, an electrically non-conductive jacket or sheath wouldnecessarily cover the heat transfer material to prevent electric shock.Jacket 20 serves as both the heat transfer material and the jacket,since jacket 20 is electrically non-conductive, but thermallyconductive.

The thermally conductive, electrically non-conductive jacket of thepresent invention approaches the conductivities of the prior art heattransfer materials. Additionally, the thermally conductive, electricallynon-conductive jacket is flexible at temperature exposures up to andexceeding 450° F. and does not harden or adhere to the substrate.

It is to be understood that the thermally conductive, electricallynon-conductive jacket replaces the prior art heat transfer cementmaterial and the silicone rubber jacket of the prior art rail heatersthat was required in certain instances due to the electricallyconductive prior art heat transfer materials using carbon fillers.Jacket material according to the present invention also replaces theprior art heat transfer cement material used without the silicone rubberjacket. As stated previously, the material of the present invention isan ideal solution for dissipating heat from a high wattage (10 to 70W/ft) rail heater. This material has a non-electrically conductivesurface with a highly thermal conductive capability. Due to the retainedflexibility of the jacket material, this material allows rail heaters tobe reusable.

With reference to FIG. 2, a typical rail 30 is heated by a heater cable32, which is made according to the present invention. Heater cable 10 ofFIG. 1 would be suitable for use as heater cable 32 in FIG. 2. Clip 34secures heater cable 32 to rail 30. Heater cable 32 has wires orconductors 32 a and 32 b and a jacket 32 c, according to the presentinvention. Where rail 30 joins another rail (not shown), heater cable 32can have an expansion loop that extends away from the rail. Suchexpansion loops would be electrical-shock hazards, except heater cable32 is covered with non-electrically conductive, but thermallyconductive, heat transfer material 32 c. Thus, a human or an animal canbrush against the expansion loop without electrical shock.

Heater cable 32 is covered with a thermally conductive material so thatheat generated by its electrical conductors is readily transferred torail 30. An electrical power source (not shown) is connected to heatercable 32 for providing electrical current for generating heat fromheater cable 32. Heater cable 32 does not become hard or brittle, butinstead remains flexible during service, even at temperatures as high as450° F. Thus, a rail may be replaced, and heater cable 32 may be reused.

The jacket material of the present invention is uniquely suitable for awide range of applications. For example, this new material can, in manyinstances, replace prior art materials (of the '458 patent) because thejacket material of the present invention has a high conductivity leveland is removable and reusable. The present material does not tend toadhere to the heated surface as did the prior art material. Thus, a heattransfer material has been discovered that is electricallynon-conductive, while also having advantageous properties includingretained flexibility and tendency to not adhere to the heated surfaceduring service.

In FIG. 3, a process pipe 40 carries a process fluid (not shown) that isto be heated according to the present invention. A tube 42 carries aheat transfer fluid, such as steam. A heat transfer material 44, whichhas a composition according to the present invention, surrounds tube 42.A channel 46 covers heat transfer material 44. Channel 46 is strapped orbanded to process pipe 40, but the straps or bands are not shown.

Heat transfer material 44 can be extruded around tube 42 as tube 42passes through a die in an extruding machine. Heat transfer material 44can also be extruded without tube 42, in which case it would beinstalled later around tube 42, such as by providing a longitudinal slitin heat transfer material 44. Tube 42 can then be inserted within heattransfer material 44 through the slit. Heat within tube 42, such asprovided by steam, passes readily through heat transfer material 44,which is thermally conductive. However, heat transfer material 44 iselectrically non-conductive. Heat transfer material 44 does not tend tobond to pipe 40, and it remains flexible after use. Thus, tube 42, withmaterial 44, can be removed from pipe 40 and reused elsewhere.

Likewise, the jacket material of the present invention is well suitedfor use with external steam/fluid panel heaters, which are a sheet ofmaterial that serves as a heat exchanger for heating a tank, vessel orthe like. With reference to FIGS. 4 and 5, a metal panel 50 transfersits heat to a vessel wall 52 through a molded sheet 54 of heat transfermaterial of the present invention. The advantage of the heat transfermaterial is, again, high heat transfer rates while achieving the uniqueflexibility, removability, and reusability of the panel.

Turning to FIG. 6, heat transfer material of the present invention willin many instances replace the present practice of welding of skin-effectheat tubes to process pipes. A skin-effect heat tube 60 is banded to aprocess pipe 62 by straps 64. A heat transfer material 66 according tothe present invention is sandwiched between tube 60 and pipe 62. Theadvantage of the present invention is that the new jacket materialcreates a thermal bridge between tube 60 and pipe 62 without physicallybonding the tube to the pipe or without the heat transfer materialcure-hardening. In the past, prior art materials have not been used dueto the potential cracking problems caused by differential thermalexpansion between the heat tube and the process pipe. Since heattransfer material 66 remains flexible and does not cure harden, it canaccommodate expansion forces without cracking.

It is anticipated that the new level of flexibility and relatively highconductivity of the jacket material of the present invention will beuseful in the construction of electric heat tracing cables. The presentheat transfer material will enable self-regulating and power limitingheaters to reduce core temperatures and thus increase power output for agiven resistivity level. Such heaters will allow constant wattage wireelements to be surrounded by the heat transfer material of the presentinvention, which serves as electrical insulation with a thermalconductance that allows an increase in the maximum current levels atwhich these heaters can safely operate.

The claimed thermally conductive, electrically non-conductive heattransfer material can be molded onto steam tracing tubes or other heattransfer surfaces, or molded into shapes to be placed on or between heattransfer surfaces, to provide a heat transfer rate in substantially thesame range as prior art heat transfer materials of 0.60 to 5.0BTU/hr-ft-° F. for ⅜″ to ¾″ outside diameter tubes while remainingremovable and reusable in service.

The heat transfer material of this invention has a substantially reducedelectrical conductivity over prior art materials, with an electricalresistivity of 1011 Ohms-cm or higher service. The heat transfermaterial is removable, reusable and stable to temperature levels inexcess of 450° F. Even after extended operation, the heat transfermaterial does not cling to or adhere to the underlying heated substrate.

The heat transfer material of this invention may be used as newelectrical insulation for self-regulating, power limiting, and constantwattage electric heat tracing cables. Self-regulating cables havedemonstrated increased power output by 7% or more with reduced operatingcore temperatures. The material utilizes a flexible heat transfer stripmaterial, which may be operated over the range of −60° F. to 450° F. toform thermal bridges between heaters and the heated substrate. The heattransfer material of this invention has greatly improved burn resistanceover prior art carbon loaded conductive materials.

EXAMPLE 1

A TEK 3C40 BN cable sample (from Thermon Manufacturing Co. of SanMarcos, Tex., U.S.A.) with a conductive silicone jacket according to thepresent invention was tested on an 85 lb. composite rail. The purpose ofthis study was to investigate the heat transfer characteristic of thethermally conductive silicone jacketed cable and compare results with aregular (thermally non-conductive) silicone jacketed cable. The cablehas an electrically conductive braid of copper wires and a jacketcovering the wires, such as shown in FIG. 1.

An 8.83 foot long TEK 3C40 BN cable sample with thermally conductivesilicone jacket was tested on a 8.66 foot long 85 lb. composite rail. Acontrol cable sample (regular silicone jacket) of identical length wasalso tested. Cable samples were tested at 5, 10, 15, and 20 watts perfoot at ambient temperature of approximately 5° F. The rail assembly wastested in a cold chamber.

Very significant temperature reductions (over standard siliconeformulations) have been achieved with the new conductive siliconeformulation as shown in FIG. 7. The lower the temperature differencebetween the braid and the exterior of the jacket, the more thermallyconductive is the jacket because the jacket efficiently transfers heatfrom the braid. FIG. 7 shows that the jacket of the present invention ismore thermally conductive than a silicone rubber that does not havefillers according to the present invention.

EXAMPLE 2

A RDT 40-600 BN cable sample from Thermon Manufacturing Co. with athermally conductive silicone jacket was tested on an 85 lb. compositerail. The heat transfer characters of thermally conductive siliconejacketed cable was compared to that of a regular silicone, SureFlow(SFOJ), jacketed cable.

An 8.83 foot long RDT 40-600 BN cable sample with conductive siliconejacket was tested on a 8.66 foot long 85 lb. composite rail. A controlcable sample (regular BNSF jacket) of identical length was also tested.J-type thermocouples were located on the cable, jacket, and rail. Cablesamples were tested at 5, 10, 20, 30 and 40 watts per foot at an ambienttemperature of approximately −6° F. The rail assembly was tested in acold chamber. The sheath and jacket temperatures were also measured forcable away from the rail. Test results are summaries in Tables 1 and 2.The temperature difference or delta T between the sheath and the jacketis lower for DT 40-600 BN thermally conductive silicone than for RDT40-600 BNSFOJ when measured on the rail. The thermally conductivejacketed (RDT 40-600) cable runs much cooler (when it is away from therail) than the cable sample jacketed with the SureFlow (SFOJ). TABLE 1W/ft Vs. DeltaT for RDT 40-600 BNSFOJ Cable on Rail Average BraidAverage Jacket Temperature Temperature T = for Cable on for Cable onT_(braid) − T_(jacket)) W/Ft the Rail in ° F. the Rail in ° F. ° F. 9.9035.23 31.80 2.57 20.25 78.90 72.42 6.47 30.41 121.22 111.85 9.37 40.82162.65 148.27 14.37

TABLE 2 W/ft Vs. Delta T for RDT 40-600 Conductive Silicone JacketedCable on Rail Average Braid Temperature Average Jacket for Cable onTemperature for Cable on T = T_(braid) − Tj_(acket)) W/Ft The Rail in °F. the Rail in ° F. ° F. 10.03 29.62 27.02 2.60 20.13 68.32 63.72 4.6029.75 110.42 103.97 6.45 40.23 154.20 145.75 8.45

EXAMPLE 3

Volume resistivity for an insulating material is used to predict thedielectric breakdown of the materials. Volume resistivity was determinedfor conductive silicone of the present invention, silicone rubber, andgraphite loaded heat transfer cement.

Volume resistivity was measured on test samples per ASTM standard D257.A Model 1864 megOhm meter manufactured by General Radio was used forvolume resistivity measurements on test samples. Terminal 1 was tied tothe − unknown terminal, terminal 2 to the guard, and terminal 3 to the +unknown terminal. Volume resistance was measured at 500 volts forthermally conductive silicone and silicone rubber samples. Volumeresistance for graphite loaded heat transfer cement was measured at 60volts because volume resistance could not be measured at 500 volts asheat transfer cement was too conductive for this measurement at voltageabove 70 volts.

Volume resistivity was calculated from measured volume resistance inOhms, the effective area of the measuring electrode in cm2, and averagethickness of the specimen in cm. Table 3 summarizes the test results.

The volume resistivity for the thermally conductive silicone is of theorder of 1012 Ohms-cm. The volume resistivity for the thermallyconductive silicone is nearly equal to that measured for siliconerubber. The volume resistivity for thermally conductive silicone is 105times greater than for graphite loaded heat transfer cement.

The thermally conductive heat transfer material of the present inventionexhibits essentially the same volume resistivity as regular, untreatedsilicone rubber, and it exhibits significantly greater volumeresistivity than prior art heat transfer material, which is filled orloaded with graphite. Thus, the claimed material is electricallynon-conductive, having an electrical resistivity of 1011 Ohms-cm orhigher. TABLE 3 Sample Electrode Volume Sample Sample Thickness AreaResistivity No. Description (Cm) (Cm²) (Ohm - Cm) 1 Thermally 0.30 50.264.33 × 10¹² Conductive Silicone 2 Silicone Rubber 0.29 50.26  4.9 × 10¹²3 Graphite Loaded Heat 0.21 50.26  2.8 × 10⁷  Transfer Cement

EXAMPLE 4

Jacket material on high power output cable may see a very hightemperature when energized at high ambient temperature environment.Therefore, jacket material should be chosen such that it will not crackduring high temperature exposure applications. This test compared RDT40-600 BN conductive silicone jacket cable samples RDT 406-600 BNSFOJcable samples, which are made from regular silicone rubber, which aremade according to the present invention. Both cables are available fromTherman Manufacturing Co. Two one foot long RDT 40-600 BN conductivesilicone jacketed cable samples were exposed to 450° F. in an oven for aperiod of 14 days. Two control samples of RDT 40-600 BNSFOJ (SureFlowwith regular silicone jacket) were tested side-by-side at 450° F. for 14days. At the end of 14 days the oven temperature was brought to roomtemperature. Samples were removed from the oven and examined visually.

Visual inspection indicated no damage or cracking on the RDT 40-600 BNconductive silicone jacketed cable samples. However, the control samples(RDT 40-600 BNSFOJ) samples had radial cracks all along the samples.Thus, RDT 40-600 BN conductive silicone jacketed cable will not crackwhen exposed to 450° F., but RDT 40-600 BNSFOJ will crack when exposedto 450° F. RDT 40-600 BN conductive silicone jacketed cable will retainflexibility even after exposure to 450° F., but RDT 40-600 BNSFOJ willlose flexibility when exposed to high temperatures. This test indicatesthat an article covered with the heat transfer material of the presentinvention will retain its flexibility after an extended period inservice at temperatures as high as 450° F.

EXAMPLE 5

This power output and temperature characteristics of thermallyconductive silicone jacketed self-regulating cable samples wereexamined. Two foot long self-regulating (VSX 20-2) bare cable was testedin an environmental chamber at 50° F. ambient. Type J thermocouples wereattached on the cable sample to measure sheath temperature. The cablewas energized at 240 volts and a stable current was recorded aftertwenty minutes. Voltage and amperage were recorded with a Beckman 4410meter and with an amp clamp. Thereafter, the same cable sample wasjacketed with a thermally conductive silicone of the present invention,and power output and temperature measurements were performed in theenvironmental chamber at an ambient of temperature of 50° F. Finally,thermally conductive silicone was removed from the sample and athermally non-conductive (regular) silicone was jacketed over the cable.Power output and sheath temperature were again measures at 50° F. in thechamber. Test results are summarized in Table 4. TABLE 4 DC ResistanceMeasured Stable Sheath Cable Type at 72° F. Voltage Amperage PowerOutput Temperature And (Length) (Ohms) (Volts) (Amps) (W/Ft) (° F.) VSX20-2 Bare 520 238 0.183 21.8 149.0 (2 foot) VSX 20-2 with 530 239 0.19523.3 140.8 80 mil silicone conductive jacket (2 foot) VSX 20-2 withregular 528 238 0.175 20.8 162 non-conductive 80 mil Silicone jacket (2foot)

Power output increased by 7% from bare to thermally conductive siliconejacketed cable. The cable runs cooler by at least 8° F. with a thermallyconductive silicone jacket as compared to the bare cable sample. Poweroutput increased by 12% between the thermally conductive siliconejacketed cable versus the regular, thermally non-conductive, siliconejacketed cable sample. Thermally conductive silicone jacketed cablesample runs 20° F. cooler than regular silicone jacketed cable sample.Thus, the power output and temperature characteristics of a jacketaccording to the present invention is better than that of either a bare,un-jacketed heater or a theater having a conventional silicone jacket.

EXAMPLE 6

The overall heat transfer conductance of a thermally conductive siliconestrip was compared to a thermally non-conductive silicone heat transferstrip. Each was extruded onto ⅜″ O.D. copper tubing and installed on anoil-filled 8″ steel pipe. Two tube strip profiles were installed undergalvanized steel channel, such as illustrated in FIG. 3. Stainless steelbanding and crimp type seals were used.

Thermocouples were placed on the steam tube at the supply location aswell as at the tracer end after exiting the pipe and insulation. Allfour tracer thermocouples were insulated with 1″ fiberglass blanket wrapto ensure accuracy of readings. In addition, thermocouples were locatedat 90° and 180° away from the tracer and at three locations along thelength of the 10 foot long pipe as shown in the below detail. Eachtracer was singly supplied with 150 psig steam and allowed to heat up toreach equilibrium prior to the temperature data being recorded.

The approximate overall conductance of the thermally conductive siliconefor a ⅜″ O.D. tube was calculate at 0.804 BTU/hr-ft-° F. and thenon-conductive silicone was calculated to be 0.434 BTU/1 hr-ft-° F.Thus, the thermal conductivity of a heat transfer material according tothe present invention is greater than that of regular silicone rubber.While prior art heat transfer materials have a heat transfer rateranging between about 0.60 and about 5.0 BTU/hr-ft-OF for tubes havingan outside diameter of ⅜″ to ¾″, the thermally conductive silicone had acalculated heat transfer rate of about 0.80 BTU/hr-ft-° F.

In summary, a thermally conductive, but electrically non-conductive,heat transfer material is provided. The heat transfer material comprisesa polymeric material, such as silicone rubber, and a nitride or oxidecompound. The nitride or oxide is preferably selected from a group orcompounds including aluminum nitride, boron nitride, silicon nitride,aluminum oxide and beryllium oxide. The nitride or oxide rangespreferably, but not absolutely necessarily, between about 30% and about60% by volume. A plasticizer is preferably added in an amount of lessthan or equal to about 15% of the weight of the nitride or oxide foradding flexibility.

The heat transfer material can be used in various articles including ajacket, covering or insulation layer for heating cables. One applicationis for rail heating, while others include electric heating and powercables, jacketed steam/fluid tracer tubes, and removable/reusablethermal bridge strips for heat tracing tubes. Articles made according tothe present invention are mechanically sturdy, while remaining flexibleafter use. Articles made with the present heat transfer material do notpose an electrical shock as the material has dielectric properties. Sucharticles do not become hard and brittle after use and do not adhere tothe heated surface. The material and articles made therefrom have thesedesirable properties, as well as meeting thermal conductivityrequirements.

A thermally conductive, electrically non-conductive heat transfermaterial that can be molded onto steam tracing tubes or other heattransfer surfaces or molded into shapes to be placed on or between heattransfer surfaces is provided. The material provides a heat transferrate in substantially the same range as prior art heat transfermaterials of 0.60 to 5.0 BTU/hr-ft-° F. for ⅜″ to ¾″ outside diametertubes and remains removable after an extended period in service. Thematerial is thus reusable after the period in service. Thus, a removableand reusable heat transfer material that is stable to temperature levelsin excess of 450° F. and does not adhere or cling to the heatedsubstrate in operation is provided.

A heat transfer material having an electrical resistivity of 1011Ohms-cm or higher is provided. An electrical insulation material forself-regulating, power limiting, constant wattage, and series resistanceelectric heat tracing cables as well as power conductors/cables isprovided. The electrical insulation material has increased power outputfor self-regulating cable by approximately 7% or higher with reducedoperating conductor/element temperatures. A flexible heat transfer stripmaterial which may be operated over the temperature range ofapproximately −60° F. to 450° F. to form a thermal bridge between aheater and the heated substrate is also provided.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the detailsof the illustrated apparatus and construction and method of operationmay be made without departing from the spirit of the invention.

1. A heating cable, comprising: a first electrical conductor, a secondelectrical conductor, and an extruded jacket covering said first andsecond electrical conductors, wherein the composition of said jacketcomprises: (1) between about 40% and about 70% by volume polymericmaterial, and (2) between about 30% and about 60% by volume of a nitrideor oxide filler material.
 2. The heating cable of claim 1, wherein thepolymeric material comprises silicone rubber.
 3. The heating cable ofclaim 1, wherein the polymeric material comprises a polyolefin.
 4. Theheating cable of claim 1, wherein the polymeric material is selectedfrom the group consisting of silicone rubber, silicone gels,polyethylene, polypropylene, elastomer, natural rubber, synthetic rubberand epoxy.
 5. The heating cable of claim 1, wherein the filler materialcomprises aluminum nitride.
 6. The heating cable of claim 1, wherein thefiller material is selected from the group consisting of aluminumnitride, boron nitride, silicon nitride, album oxide and berylliumoxide.
 7. The heating cable of claim 1, wherein the filer materialcomprises between about 40 percent and about 50 percent by volume of thejacket.
 8. The heating cable of claim 1, wherein the polymeric materialcomprises silicone rubber and the filler material comprises aluminumnitride.
 9. The heating cable of claim 1, wherein the polymeric materialand the filler material are present in amounts resulting in the jackethaving an electrical resistivity of about 10¹¹ Ohms-cm or higher. 10.The heating cable of claim 1, wherein the heating cable comprises aself-regulating cable.
 11. The heating cable of claim 1, wherein theheating cable comprises a power limiting cable.
 12. The heating cable ofclaim 1, wherein the heating cable comprises a constant wattage cable.13. The heating cable of claim 1, wherein the heating cable comprises aflexible cable.
 14. The heating cable of claim 1, wherein said jacketfurther comprises a plasticizer up to and including about 15% of theweight of the nitride or oxide.
 15. An apparatus for heating railroadrails, comprising; a heating cable comprising two conductors and ajacket, wherein said jacket is extruded around said conducts and has acomposition comprising: (1) a polymeric material, and (2) a nitride oroxide filer material; and at least one clip for se said heating cable toa rail.
 16. The apparatus of claim 15, wherein said jacket issubstantially non-adhering to said rail.
 17. The apparatus of claim 15,wherein said heating cable is removable and reusable.
 18. A heatingcable, comprising: a first electrical conductor, a second electricalconductor; and an extruded jacket covering said first and secondelectrical conductors, wherein the composition of said jacket comprises:(1) silicone rubber, and (2) oxide filler.
 19. The heating cable ofclaim 18, wherein the polymeric material and the filler material arepresent in amounts resulting in the jacket having an electricalresistivity of about 10¹¹ Ohms-cm or higher.
 20. A heating cable,comprising: a first electrical conductor; a second electrical conductor;an extruded jacket covering said first and second electrical conductors,wherein the composition of said jacket comprises: (1) a polymericmaterial, and (2) aluminum nitride.
 21. The heating cable of claim 20,wherein the polymeric material comprises silicone rubber.
 22. Theheating cable of claim 20, wherein the polymeric material and the fillermaterial are present in amounts resulting in the jacket having anelectrical resistivity of about 10¹¹ Ohms-cm or higher.
 23. A heatingcable, comprising: a first electrical conductor; a second electricalconductor; and an extruded jacket covering said first and secondelectrical conductors, wherein the composition of said jacket Comprises:(1) polymeric material, and (2) a nitride or oxide filler material, andwherein the polymeric material is selected from the group consisting ofsilicone rubber, silicone gels, polyethylene, polypropylene, elastomer,natural rubber, synthetic rubber and epoxy.
 24. The heating cable ofclaim 23, wherein the polymeric material and the filler material arepresent in amounts resulting in the jacket having an electricalresistivity of about 10¹¹ Ohms-cm or higher.
 25. A heating cable,comprising: a first electrical conductor; a second electrical conductor;an extruded jacket covering said first and second electrical conductors,wherein the composition of said jacket comprises (1) polymeric material,and (2) between about 30% and about 60% by volume nitride or oxidefiller material.
 26. The heating cable of claim 25, wherein the fillermaterial comprises between about 40 percent and about 50 percent byvolume of the jacket.
 27. A heating cable, comprising: a firstelectrical conductor; a second electrical conductor; and an extrudedjacket covering said first and second electrical conductors, wherein thecomposition of said jacket comprises: (1) polymeric material, and (2) anitride or oxide filler material and wherein the polymeric material andthe finer material are present in amounts resulting in the jacket havingan electrical resistivity of about 10¹¹ Ohms-cm or higher.
 28. Theheating cable of claim 27, wherein the polymeric material comprisessilicone rubber and the filler material comprises aluminum nitride. 29.The heating cable of claim 28, wherein the aluminum nitride comprisesbetween and about 30 percent and about 60 percent by volume of thejacket and the silicone rubber comprises between about 40% and about 70%by volume of the jacket.
 30. A heating cable, comprising: a firstelectrical conductor; a second electrical conductor; and an extrudedjacket covering said first and second electrical conductors, wherein thecomposition of said jacket comprises: (1) polymeric material and (2) anitride or oxide filler material, and wherein the heating cablecomprises a power limiting cable.
 31. A heating cable, comprising: afirst electrical conductor; a second electrical conductor; and anextruded jacket covering said first and second electrical conductors,wherein the composition of said jacket comprises: (1) polymericmaterial, and (2) a nitride or oxide filler material, and wherein theheating cable comprises a constant wattage cable.